1. Field of the Invention
The invention relates to pressure relief and more particularly to a pressure relief device for high volume low pressure steam service.
2. Discussion of the Prior Art
In making cellulose pulp, large quantities of low pressure steam are generated when the contents of a digester under high pressure are flashed to atmospheric pressure in a blow tank. High energy costs and concern for pollution control make it necessary to condense all of this steam without allowing any to escape to the atmosphere. Generally, the vessels downstream of the digester are not pressure vessels and have to be protected by means of pressure relief valves. The type of valve most commonly used for pressure relief in such applications is a counter-balanced flapper valve, but leakage and the corrosion of mechanical linkages in such valves make them undependable and less than entirely satisfactory.
In a typical application, the pressure relief valve opens only two or three times a year, at the maximum. Normal operating pressures are 3 pounds per square inch gauge internal and 1 pound per square inch gauge external pressure and the steam flow can reach peak loads of 900,000 pounds of steam per hour. A highly dependable, leak proof valve is clearly desirable.
Although mechanical valves are used for pressure relief in pulp making, it should be noted that various kinds of water seals have been employed in relatively low steam volume applications in the past without really satisfactory results.